It is normal, particularly in telecom systems, for a DC power system to be provided, with the load being connected across the DC voltage. Other systems that have critical loads may, of course, be connected across AC systems.
In either event, where the load is a critical load such as components of a telecom system, the load is protected by a circuit breaker or fuse which is in series with it across the voltage system. Occasionally, the load may be between a pair of ganged circuit breakers. The circuit breaker or fuse is provided and is sized so that it will open if the load current through the load--and, of course, through the circuit breaker or fuse--exceeds the predetermined current handling capacity of the circuit breaker or fuse.
In the following discussion, the words "circuit breaker" and "fuse" are used essentially interchangeably, and indicate a device which is designed and intended to open under a predetermined current condition to protect the load with which it is in series.
Of course, if the circuit breaker opens for whatever reason, the operator of the system wants to have some alarm indication of the fact that the circuit breaker has opened. Remedial action may be taken, or the load may be examined to determine why it suddenly required a higher current than normal.
Thus, the use of a circuit breaker protects the load, and the generation of some kind of signal is required to indicate to the operator that the circuit breaker has opened to protect the load. The usual arrangement has been the use of auxiliary contacts or indicating fuses which are physically located in the circuit breaker; usually in such a manner that the auxiliary contacts are open when the circuit breaker is closed, and the auxiliary contacts are closed when the circuit breaker opens. The closing of the auxiliary contacts makes another circuit which is independent of the load although it may be across the same voltage system, and in that other circuit an alarm signal generating means is provided.
However, the quality control, and indeed the design, of auxiliary contacts is such that it can not always be assured that the auxiliary contacts will make--that is, they will close--when they should. If that is the case, then the circuits which rely on the operation of the auxiliary contacts are neither trustworthy nor fail safe.
It should also be noted that, particularly in telecom circuits, the circuit breaker which protects the critical load is arranged only at one side of the system. Usually, the positive bus of a DC system is grounded, and the load is placed between the positive and negative sides in series with the circuit breaker which is at the negative end of the load. In a central switching station for such as a telephone system, many hundreds of mechanical circuit breakers with their auxiliary contacts may be used; and clearly, it is less than satisfactory for there to be less than 100% certainty that failure of any critical load and the opening of a circuit breaker to protect that load, will produce a signal which signifies that fact.
In some circumstances, usually higher voltage systems, the negative side of the system may be grounded. Under other circumstances, the voltage system may be an alternating current system, usually with the neutral side of the system connected to ground. In still other circumstances, the system may be operating as an ungrounded or floating direct current system; and in that case, it is usual for the load to be protected at each side by a breaker which is ganged or connected such as through a double pole tie to the other breaker--so that if the one breaker opens, the other breaker will also open.
What the present invention provides is an alarm circuit which directly monitors opening of the circuit breaker itself, not the mechanical auxiliary contacts or indicating fuse that might be used in association with the circuit breaker. Moreover, the present invention provides such an alarm circuit which is fully solid state, thereby precluding any possibility of mechanical failure.
Therefore, the present invention provides an alarm circuit which is arranged to provide a status or alarm signal at least when the circuit breaker in series with a critical load across a voltage system has opened. The alarm circuit includes a high resistance resistor which is arranged to drive a high gain amplifier, which functions as a solid state switch; the circuit being arranged therefore as a status monitor. The high resistance resistor is connected at its first end to the load end of the circuit breaker, and at its second end to the input of the high gain amplifier. In turn, the high gain amplifier is connected at its output to one side of a remote signal relay or switch which may be solid state such as an SCR or triac, or it may be a conventional relay. The other side of the remote signal relay is connected to one side of the voltage system. The connection of the high resistance resistor to the input of the high gain amplifier is such that the high gain amplifier is maintained in a substantially non-conductive condition. As described hereafter, if the breaker opens, then the high gain amplifier changes its state to become conductive--in another words, its output goes from low to high. If the remote signal relay receives a high output from the high gain amplifier, which is indicative of the circuit breaker having opened, then the remote signal relay will change its state. If so, then means are associated with the solid state relay to provide a signal which is indicative of the change of state of that relay. Since the remote signal relay will not change its state unless the circuit breaker opens, then the signal is indicative of the fact that the circuit breaker has opened.
More particularly, the high gain amplifier itself functions as a solid state switch, driving another relay--which is the remote signal relay discussed above. Still further, the circuits of the present invention provide for both a local alarm and a remote alarm. The local alarm is generally in the form of an LED in the circuits, and the remote alarm takes its signal from the remote signal relay so that it is isolated from the alarm circuits of the present invention, but operative with them. The LED is in series with the output of the high gain amplifier, so that when the high gain amplifier becomes conductive, the LED becomes illuminated.
As will be described in greater detail hereafter, the remote signal relay may be in series with the high gain amplifier, or it may be in parallel (shunt) with the output of the high gain amplifier. Moreover, as noted, the alarm circuits of the present invention may be adapted to work with a grounded or a floating DC voltage system, or an AC voltage system; and in an AC voltage system the remote signal relay may be an AC relay or a DC relay.
By using a solid state alarm circuit in keeping with the present invention, a much higher inherent reliability is assured. Obviously, the MTBF (Mean Time Between Failures) rating of a resistor or a transistor, (or an FET functioning as a high gain amplifier or as a solid state relay), is much higher than the MTBF rating of mechanical auxiliary contacts or even mechanical relays. Because of the arrangement of the present invention, it is the circuit breaker itself which is monitored by the alarm circuit, and not the auxiliary contacts which heretofore have been monitored by alarm circuits especially in telecom systems. The present invention assures that in all events a local alarm indication (the LED) is made when the circuit breaker opens, and it assures that by using a remote signal relay at the output of its solid state relay or high gain amplifier that a remote alarm may be isolated from but driven by the present alarm circuits.